1. Field of the Invention
This invention relates to gearing, and in particular to the profile of the gear tooth and method of generating same.
2. Prior Art
Considerable effort has been expended in the development and improvement of the standard involute tooth form. As a result, it has arrived at a point in its developmental life where significant improvements in capacity are extremely difficult and costly to achieve. In conjunction with and in many ways as an outgrowth of this development a variety of gear tooth forms have been presented in recent years, for example, the Cycloidal, Wildhaber-Novikov (W-N), the high sliding (HSG), the Evoloid, the high profile contact ratio (HCR), and the constant relative curvature (CRC). Each of these forms has its own advantages and disadvantages. The Cycloidal, (W-N) and (CRC) forms are truly non-involute, while the (HSG), (HCR) and Evoloid are essentially modifications to the basic involute. The Cycloidal and the (W-N) forms have been demonstrated to have limitations which, except for a very few unusual applications, make them generally unsuitable for high speed, high power applications. The (HSG) and the Evoloid forms are essentially similar in their basic concepts, i.e., each is a systematized method for extreme profile shifting utilizing standard involute profiles. Profile shifting, while relatively new to commercial applications, has been utilized for some time in the design of high capacity precision systems such as aircraft and helicopter transmissions. The (HSG) and Evoloid forms both shift profiles to such an extent that the resultant profile contact ratio is quite low, in fact, in many cases it falls substantially below unity. Because of the low profile contact ratio, both forms must generally be made as helical gears, thus introducing a thrust force which requires an additional bearing to be reacted. In addition, both gear forms have very high sliding velocities along the tooth profile. High sliding generally indicates both high scoring probability and lower surface fatigue life. Both of these forms provide a lower noise level and high reduction ratios than are generally practical with standard involute gears. However, their limitations as noted above are sometimes considered to outweigh these advantages.
The (HCR) form utilizes a basic involute form but changes are made to the addendum area, the tooth root, whole depth, pressure angle, etc., to increase the profile contact ratio from 1.2-1.35 which is typical of the standard involute gearing to 2 and above. The advantages of (HCR) gearing include lower stress levels per unit load (thus permitting greater loads to be carried without exceeding basic stress limits), lower noise and vibration levels, improved reliability, and generally smoother mesh action.
While the (HCR) profile has the advantages noted above, a major disadvantage is in the somewhat higher sliding velocities and slip ratios which occur, particularly in the pinion dedendum area, due to the extended contact length. The increased sliding gives rise to a real scoring hazzard to the gear tooth.
The (CRC) form is not generated from a base circle as is the involute tooth form which results in a constantly changing radius of curvature of the tooth profile. In the (CRC) form the radius is substantially constant as disclosed in U.S. Pat. No. 3,631,736. In this patent the three stated objects relate the desire to provide an improvement of the basic involute form in terms of its strength characteristics. The (HCR) form and its relationship to the basic involute form as a means of strength improvement are not mentioned although the (HCR) form was known in the relevant time period of this patent.